I decided to benchmark this and see what the difference was. Here is the script...

#!/usr/bin/perl

use strict;
use warnings;

use Benchmark qw(:all);
use Params::Validate qw(:all);

sub ParamsValidateAssert {
    my ($test) = validate_pos( @_,
                        { 
                            type => SCALAR,
                            callbacks => { 'must be less than 50' => s
+ub { $_[0] < 50 }},
                        }
                    );
    return $test;
}

sub OrAssert {
    my ($test) = @_;
    ($test < 50) || die "Test is wrong";
    return $test;
}

my @nums = map { ((rand() * 100) % 50) } (0 .. 100);

cmpthese(10_000, {    
    'ParamsValidateAssert' => sub { eval { ParamsValidateAssert($_) } 
+for (@nums) },    
    'OrAssert'             => sub { eval { OrAssert($_)             } 
+for (@nums) },   
});
[download]

Benchmark: timing 10000 iterations of OrAssert, ParamsValidateAssert..
+.
  OrAssert:  8 wallclock secs ( 7.02 usr +  0.01 sys =  7.03 CPU) @ 14
+22.48/s (n=10000)
ParamsValidateAssert: 97 wallclock secs (87.88 usr +  0.38 sys = 88.26
+ CPU) @ 113.30/s (n=10000)
                       Rate ParamsValidateAssert             OrAssert
ParamsValidateAssert  113/s                   --                 -92%
OrAssert             1422/s                1155%                   --
[download]

But as water points out below, it should not completely be about the speed. I am sure that Params::Validate has a number of wheels I would not want want to have to re-invent when I hand-code OR based assertions. Looking over the docs for Params::Validate, I see where this could really be a useful tool for data validation, not just of subroutine parameters, but of web queries and such. As you say it can be quite declarative and I can see how you could build some very sophisticated data validation with it. However, I do think it is overkill for subroutine params and post/pre-condition checking.